Ultrasonic examinations are performed within the nuclear industry and most other major industries to determine the condition of parts and components. The metal or alloy material of a part or component is inspected using ultrasound to detect any flaws which could prove detrimental to the safe operation of that part or component. The ultrasonic nondestructive examination method can be used to detect internal flaws in most engineering metals and alloys. Bonds produced by welding, brazing, soldering and adhesive bonding can also be ultrasonically inspected.
Ultrasonic inspection is used for quality control and materials inspection in the fabrication of structures, reactor pressure vessels, airframes, pipe systems, bridges, motor vehicles and jet engines. The present invention has application in all of these fields.
For successful application of ultrasonic examination techniques, methods and equipment, the ultrasonic system, including transducers, must be suitable for the type of inspection being performed. If the correct transducer is not used, there is a high potential for gross error in the inspection results, or there could be no results at all. For instance, using a common ultrasonic transducer that has a hard flat-surfaced LUCITE wedge for examining as-welded overlaid pipe welds results in gross errors in the ultrasonic inspection results. In many cases ultrasonic inspection data is not recorded at all. This is due to the presence of air gaps between the transducer head and the rough surface being inspected, which form an opaque barrier.
Ultrasonic characterization of cracks in materials is at least a two-step process: 1) detection and location; and 2) sizing in absolute or relative terms. In accordance with the first step of this process, the transducer is excited to emit a longitudinal ultrasonic wave which is coupled to the structure being inspected. The emitted wave enters the structure, where it is reflected by the crack. The return path of the reflected wave impinges on the transducer, where it is detected as a "pulse echo" signal.
The determination of the crack size, or depth of penetration in the case of surface-connected flaws, is a different and more complicated task. A conventional method for determining the depth of penetration of a planar crack is the time-of-flight diffraction technique. This method takes advantage of the forward scattering of waves of ultrasonic energy at the edges of a crack. An emitter of short pulses of ultrasound, coupled to the inspection surface, causes refracted sound waves to impinge on the crack edge, which scatters the ultrasonic energy in all directions. A detector situated on the opposite side of the crack plane is excited by the ray of scattered pulsed energy after a time delay that is a function of the crack height and other dimensions. By measuring the time-of-flight of the pulses from the emitter to the detector by way of the crack edge, the crack height can be easily computed from the geometry.
Inspection methods using the ultrasonic time-of-flight diffraction technique have been devised for buried, as well as surface-connected, cracks and have proven to be the most accurate means of crack sizing in practice. Corrections for surface curvature effects are employed for use with pipes and nozzles, where necessary, to enhance accuracy. On the other hand, the method clearly fails if the scattered wave is unable to reach the detector, which occurs if there is a relatively non-uniform gap interposed between the transducer and the surface being inspected. Such gaps arise when a hard, flat-surfaced ultrasonic transducer overlies an uneven or rough sound beam entry surface, such as a pipe weld in as-welded condition, i.e., without post-weld machining.
Booted transducers are known to have existed within the nondestructive examination industry in the past. One known design contains relatively small transducers installed within a large plastic case. The coupling medium used in that prior art booted transducer is a low-viscosity compressor oil. The angle of incidence for the ultrasonic wave produced by the transducers is determined by a holding bracket installed inside the plastic case on which the transducers are mounted. After installation of a holding bracket corresponding to the desired angle of incidence, the transducer is installed, the plastic boot is fixed in place and the whole assembly is filled with oil.
The foregoing type of booted transducer tends to be extremely large and its contact footprint often is too large for the part to be examined. Another limitation of this prior art assembly is that changing the transducer is difficult. First the oil must be drained; then the boot must be removed. If the angle of incidence is to be changed, the original holding bracket must be removed and another bracket corresponding to a different angle of incidence must be installed. The transducer is then installed and the whole assembly process is repeated. Very often the transducer assembly must be left idle while air bubbles rise to the surface of the oil and then are bled off. In general, this is an extremely time-consuming and inefficient process, especially when performed in hostile environments such as nuclear power plants.